Emerging flexible electronic devices have exhibited significant potential for a wide range of applications such as solar cells (1), batteries (2), sensors (3), antennas (4), and displays (5). For any flexible electronic application, an essential characteristic is electrically conductive patterning. Solution-based additive manufacturing techniques such as drop-on-demand (DOD) inkjet printing (6), slot die coating (7), and gravure printing (8) are being widely investigated to fabricate flexible conductive patterns. DOD inkjet printing is an excellent candidate because it is a material-conservative, low-temperature process and is easily incorporated into large scale roll-to-roll (R2R) manufacturing infrastructures for flexible polymer substrates.
The commonly used ink materials in DOD inkjet printing processes can be categorized into two types: metal nanoparticle (NP) dispersions and metal precursor solutions (9, 82). NP inks consist of metallic NPs and a carrier liquid solvent. The NPs have specifically designed surface properties which allow them to be stably dispersed in an appropriate solvent. Precursor ink is an inorganic metal salt or organic metal complex that is dissolved in a solvent. After inkjet printing, the solvent undergoes evaporation and metal NP or precursor deposits on the substrate. A post-printing process is typically necessary to render either type of deposited structure electrically conductive. Polymer capping layers and surfactants are employed in the formulation of NP inks to prevent aggregation and particle precipitation; these agents are generally neither conductive nor volatile. The post-printing process removes these agents and initiates sintering, thereby improving the electrical conductivity. For precursor inks, the post-printing process chemically reduces the metal species from its ionic to elemental state which is electrically conductive. The standard post-printing process applies heat that potentially puts substrate materials at risk of thermal degradation/deformation, especially when low-cost polymeric substrates are used (e.g., polyethylene terephthalate (PET), etc.). A few non-thermal or local surface thermal techniques have been reported including plasma (10, 11), laser (12), electrical (13), and photonic (14) methods. However, sophisticated equipment and their associated high-cost processes are inevitable.
Drop-on-demand inkjet printing is a material-conservative deposition process compatible with the low temperature requirements of flexible polymer substrates (81). The implementation of printing within a roll-to-roll (R2R) infrastructure enables continuous, high-speed and large-scale manufacturing.
Electroless plating generally uses a solution of metal salt, reducing agent, a complexing agent, and additive(s) (such as bath stabilizer and plating rate adjusting agent)(20). Metal nucleates on the catalytically active surface and continues to promote further metal reduction and growth. This is the defining characteristic of ELP's autocatalytic nature (20). A pre-patterned catalyst layer on the target substrate will promote site-selective deposition. Hidber et al. (86) utilized microcontact stamping to pattern palladium (Pd) colloids which yielded copper (Cu) patterns after ELP. Harkness et al. (87) used photolithography to pattern a Pd-bonded-organic seed layer (hydrogen silsesquioxane) and achieved site selective ELP of Cu and Ag. The prohibitively large cost of photolithography has prompted research into the use of inkjet printing to directly pattern the catalyst (88, 89, 90, 91, 92, 93, 94). Most of the inkjet printing studies exploited Pd-based ink (88, 89, 90, 91, 92) while the investigation of other ink materials is less focused (93, 94). Although Pd-based catalyst is versatile for electroless plating of a wide range of metals, the high cost of Pd limits its use (95).
Silver (Ag) is the most broadly investigated conductive ink material due to its low bulk resistivity (1.6×10−8 Ωm) and resistance to oxidation; however, like other noble metals, it is expensive ($0.708/gram) (15). Copper (Cu) is preferred because it exhibits a bulk resistivity (1.7×10−8 Ωm) comparable to Ag but is significantly cheaper ($0.007/gram) (15). However, the main challenge of using Cu-based raw material for inkjet printing arises from the spontaneous formation of Cu oxides; when synthesized Cu NPs oxidize, both their resistivity and sintering temperature increase dramatically (9). Research efforts to overcome the Cu NP oxidation have taken two directions: utilizing an organic oxygen barrier material as particle capping layers to retard oxidation kinetics (16) and synthesis of Cu-noble metal core-shell NPs to achieve long-term stability (17). Cu precursor inks are usually stable against oxidation under room environment (18). Notwithstanding, the postprinting process for both Cu NP and Cu precursor must be implemented in reductive, inert atmospheres or under vacuum to prevent oxide formation, which inherently increases process complexity (19). Site-selective Cu electroless plating (ELP) is a method that can be used to fabricate conductive patterns on flexible substrates. It is a low temperature process that does not cause substrate damage if a proper plating bath is used. During ELP, formation of Cu oxides is dramatically inhibited. The general ELP uses a solution of metal salt, complexing agent, reducing agent and additive(s) (such as a bath stabilizer and a pH adjusting agent) (20).
Site-selective ELP can be achieved by plating a substrate which has a prepatterned catalyst/seed layer. Studies have been conducted exploring inkjet printing for ELP seed patterning (21, 22, 23). The Pd-based ink is the most widely investigated material due to its well-established catalytic activity for initiation of various metal deposition from a wide range of ELP solutions (24).
Poly(dopamine) (PDA), a marine mussel inspired polymer, was recently found capable of initiating metal ion reduction indicating its potential as an ELP catalyst (26, 67, 96). PDA exhibits universal adhesion as demonstrated for a wide range of both organic and inorganic materials (26). PDA can be synthesized as continuous coatings on any object by inducing dopamine polymerization in a pure water phase (26) or as suspended spherical NPs in water-alcohol mixtures at controlled pH (27).
Poly(dopamine) nanoparticles (PDA-NP) have been inkjet printed on both glass and PET substrates followed by site-selective Ag ELP (28). Results exhibit a substrate-independent method to fabricate highly conductive Ag patterns.
The coffee ring effect is due to capillary flow induced by the differential evaporation rates across the drop: liquid evaporating from the contact line (“CL”) region is replenished by liquid from the interior (29). When the CL is pinned and maximum evaporation occurs at or near the CL, the mass flow of solvent causes an accumulation of solute in the CL region. The resulting bulk flow toward the CL can transport nearly all the dispersed material to the CL region. The cooling due to evaporation induces a Marangoni flow inside a droplet opposing the evaporatively-driven flow. The Marangoni flow, if strong, favors particle deposition at the center region of the droplet. Thus, for particles to accumulate in the CL region, the liquid must have a weak Marangoni flow, or something must occur to disrupt the flow. For example, surfactants can be added to reduce the liquid's surface tension gradient, disrupting the Marangoni flow. Water has an intrinsically weak Marangoni flow, a flow that is then reduced significantly by natural surfactants.